1. Field of the Invention
This invention relates to an image forming apparatus, and more particularly to an image forming apparatus in which adherence of to excess toner to a photosensitive medium is prevented.
2. Description of the Prior Art
Heretofore, in a copying apparatus having the magnification changing function, the process of preventing toner from adhering to a non-image area (the black-erasing process) during stepped magnification change could be accomplished by various means disposed in the direction of rotation of the drum, but aligned in a direction parallel to the axis of the drum. Such process has usually been accomplished by placing a tiny lamp adjacent the drum and turning the lamp on and off in accordance with a selected magnification factor. In the case of continuous magnification change, however, it has been necessary to turn the lamp on and off continuously, and with the conventional tiny lamp, it has been impossible to effect this function because of the pitch involved. Also, means comprising a plurality of light-emitting elements such as LEDs (light-emitting diodes) has been proposed, but such means has not been usable due to the deficiency of quantity of light because, in a copying apparatus using the NP process, light must be applied from behind a charger for simultaneous exposure and discharging and because the optical path of the referred light from an original must not be intercepted. There has been a further problem that such process cannot be effected in continuous magnification changes below the pitch interval of LEDs. Also, means have been proposed whereby a movable shielding member disposed in front of the lamp is continuously moved in accordance with a magnification factor to continuously intercept the light of the lamp, but such means have not yet been realized because the image on the photosensitive medium is adversely affected by diffraction or irregular reflection of light and because there occurs a problem such as complexity of the construction. Ideally, however, elements which are very small point sources of light and whose lights do not diverge and which are arranged at a very small pitch would be useful, but such ideal elements have not yet been provided.